3GPP LTE employs Orthogonal Frequency Division Multiple Access (OFDMA) as a downlink communication scheme. In radio communication systems to which 3GPP LTE is applied, base stations transmit synchronization signals (i.e., Synchronization Channel: SCH) and broadcast signals (i.e., Broadcast Channel: BCH) using predetermined communication resources. Meanwhile, each terminal finds an SCH first and thereby ensures synchronization with the base station. Subsequently, the terminal reads BCH information to acquire base station-specific parameters (e.g., frequency bandwidth) (see, Non-Patent Literature (hereinafter, abbreviated as NPL) 1, 2 and 3).
In addition, upon completion of the acquisition of the base station-specific parameters, each terminal sends a connection request to the base station to thereby establish a communication link with the base station. The base station transmits control information via Physical Downlink Control CHannel (PDCCH) as appropriate to the terminal with which a communication link has been established via a downlink control channel or the like.
The terminal performs “blind-determination” on each of a plurality of pieces of control information included in the received PDCCH signal (i.e., Downlink (DL) Assignment Control Information: also referred to as Downlink Control Information (DCI)). More specifically, each piece of the control information includes a Cyclic Redundancy Check (CRC) part and the base station masks this CRC part using the terminal ID of the transmission target terminal. Accordingly, until the terminal demasks the CRC part of the received piece of control information with its own terminal ID, the terminal cannot determine whether or not the piece of control information is intended for the terminal. In this blind-determination, if the result of demasking the CRC part indicates that the CRC operation is OK, the piece of control information is determined as being intended for the terminal.
Moreover, in 3GPP LTE, Automatic Repeat Request (ARQ) is applied to downlink data to terminals from a base station. More specifically, each terminal feeds back a response signal indicating the result of error detection on the downlink data to the base station. Each terminal performs a CRC on the downlink data and feeds back Acknowledgment (ACK) when CRC=OK (no error) or Negative Acknowledgment (NACK) when CRC=Not OK (error) to the base station as a response signal. An uplink control channel such as Physical Uplink Control Channel (PUCCH) is used to feed back the response signals (i.e., ACK/NACK signals (hereinafter, may be referred to as “A/N,” simply)).
The control information to be transmitted from a base station herein includes resource assignment information including information on resources assigned to the terminal by the base station. As described above, PDCCH is used to transmit this control information. This PDCCH includes one or more L1/L2 control channels (L1/L2 CCH). Each L1/L2 CCH consists of one or more Control Channel Elements (CCE). More specifically, a CCE is the basic unit used to map the control information to PDCCH. Moreover, when a single L1/L2 CCH consists of a plurality of CCEs (2, 4 or 8), a plurality of contiguous CCEs starting from a CCE having an even index are assigned to the L1/L2 CCH. The base station assigns the L1/L2 CCH to the resource assignment target terminal in accordance with the number of CCEs required for indicating the control information to the resource assignment target terminal. The base station maps the control information to physical resources corresponding to the CCEs of the L1/L2 CCH and transmits the mapped control information.
In addition, CCEs are associated with component resources of PUCCH (hereinafter, may be referred to as “PUCCH resource”) in a one-to-one correspondence. Accordingly, a terminal that has received an L1/L2 CCH identifies the component resources of PUCCH that correspond to the CCEs forming the L1/L2 CCH and transmits a response signal to the base station using the identified resources. However, when the L1/L2 CCH occupies a plurality of contiguous CCEs, the terminal transmits the response signal to the base station using a PUCCH component resource corresponding to a CCE having a smallest index among the plurality of PUCCH component resources respectively corresponding to the plurality of CCEs (i.e., PUCCH component resource associated with a CCE having an even numbered CCE index). In this manner, the downlink communication resources are efficiently used.
As illustrated in FIG. 1, a plurality of response signals transmitted from a plurality of terminals are spread using a Zero Auto-correlation (ZAC) sequence having the characteristic of zero autocorrelation in time-domain, a Walsh sequence and a discrete Fourier transform (DFT) sequence, and are code-multiplexed in a PUCCH. In FIG. 1, (W0, W1, W2, W3) represent a length-4 Walsh sequence and (F0, F1, F2) represent a length-3 DFT sequence. As illustrated in FIG. 1, ACK or NACK response signals are primary-spread over frequency components corresponding to 1 SC-FDMA symbol by a ZAC sequence (length-12) in frequency-domain. More specifically, the length-12 ZAC sequence is multiplied by a response signal component represented by a complex number. Subsequently, the ZAC sequence serving as the response signals and reference signals after the primary-spread is secondary-spread in association with each of a Walsh sequence (length-4: W0-W3 (may be referred to as Walsh Code Sequence)) and a DFT sequence (length-3: F0-F2). More specifically, each component of the signals of length-12 (i.e., response signals after primary-spread or ZAC sequence serving as reference signals (i.e., Reference Signal Sequence) is multiplied by each component of an orthogonal code sequence (i.e., orthogonal sequence: Walsh sequence or DFT sequence). Moreover, the secondary-spread signals are transformed into signals of length-12 in the time-domain by inverse fast Fourier transform (IFFT). A CP is added to each signal obtained by IFFT processing, and the signals of one slot consisting of seven SC-FDMA symbols are thus formed.
The response signals from different terminals are spread using ZAC sequences each corresponding to a different cyclic shift value (i.e., index) or orthogonal code sequences each corresponding to a different sequence number (i.e., orthogonal cover index (OC index)). An orthogonal code sequence is a combination of a Walsh sequence and a DFT sequence. In addition, an orthogonal code sequence is referred to as a block-wise spreading code in some cases. Thus, base stations can demultiplex the code-multiplexed plurality of response signals using the related art despreading and correlation processing (see, NPL 4).
However, it is not necessarily true that each terminal succeeds in receiving a downlink assignment control signal because the terminal performs blind-determination in each subframe to find a downlink assignment control signal intended for the terminal. When the terminal fails to receive the downlink assignment control signal intended for the terminal on a certain downlink component carrier, the terminal would not even know whether or not there is downlink data intended for the terminal on the downlink component carrier. Accordingly, when a terminal fails to receive the downlink assignment control signal intended for the terminal on a certain downlink component carrier, the terminal generates no response signals for the downlink data on the downlink component carrier. This error case is defined as discontinuous transmission of ACK/NACK signals (DTX of response signals) in the sense that the terminal transmits no response signals.
In 3GPP LTE systems (may be referred to as “LTE system,” hereinafter), base stations assign resources to uplink data and downlink data, independently. For this reason, in the 3GPP LTE system, terminals (i.e., terminals compliant with LTE system (hereinafter, referred to as “LTE terminal”)) encounter a situation where the terminals need to transmit uplink data and response signals for downlink data simultaneously in the uplink. In this situation, the response signals and uplink data from the terminals are transmitted using time-division multiplexing (TDM). As described above, the single carrier properties of transmission waveforms of the terminals are maintained by the simultaneous transmission of response signals and uplink data using TDM.
In addition, as illustrated in FIG. 2, the response signals (i.e., “A/N”) transmitted from each terminal partially occupy the resources assigned to uplink data (i.e., Physical Uplink Shared CHannel (PUSCH) resources) (i.e., response signals occupy some SC-FDMA symbols adjacent to SC-FDMA symbols to which reference signals (RS) are mapped) and are thereby transmitted to a base station in time-division multiplexing (TDM). However, “subcarriers” in the vertical axis in FIG. 2 are also termed as “virtual subcarriers” or “time contiguous signals,” and “time contiguous signals” that are collectively inputted to a discrete Fourier transform (DFT) circuit in a SC-FDMA transmitter are represented as “subcarriers” for convenience. More specifically, optional data of the uplink data is punctured due to the response signals in the PUSCH resources. Accordingly, the quality of uplink data (e.g., coding gain) is significantly reduced due to the punctured bits of the coded uplink data. For this reason, base stations instruct the terminals to use a very low coding rate and/or to use very large transmission power so as to compensate for the reduced quality of the uplink data due to the puncturing.
Meanwhile, the standardization of 3GPP LTE-Advanced for realizing faster communication than 3GPP LTE is in progress. 3GPP LTE-Advanced systems (may be referred to as “LTE-A system,” hereinafter) follow LTE systems. 3GPP LTE-Advanced will introduce base stations and terminals capable of communicating with each other using a wideband frequency of 40 MHz or greater to realize a downlink transmission rate of up to 1 Gbps or above.
In the LTE-A system, in order to simultaneously achieve backward compatibility with the LTE system and ultra-high-speed communication several times faster than transmission rates in the LTE system, the LTE-A system band is divided into “component carriers” of 20 MHz or below, which is the bandwidth supported by the LTE system. In other words, the “component carrier” is defined herein as a band having a maximum width of 20 MHz and as the basic unit of communication band. In the Frequency Division Duplex (FDD) system, moreover, “component carrier” in downlink (hereinafter, referred to as “downlink component carrier”) is defined as a band obtained by dividing a band according to downlink frequency bandwidth information in a BCH broadcasted from a base station or as a band defined by a distribution width when a downlink control channel (PDCCH) is distributed in the frequency domain. In addition, “component carrier” in uplink (hereinafter, referred to as “uplink component carrier”) may be defined as a band obtained by dividing a band according to uplink frequency band information in a BCH broadcasted from a base station or as the basic unit of a communication band of 20 MHz or below including a Physical Uplink Shared CHannel (PUSCH) in the vicinity of the center of the bandwidth and PUCCH s for LTE on both ends of the band. In addition, the term “component carrier” may be also referred to as “cell” in English in 3GPP LTE-Advanced. Furthermore, “component carrier” may also be abbreviated as CC(s).
In the Time Division Duplex (TDD) system, a downlink component carrier and an uplink component carrier have the same frequency band, and downlink communication and uplink communication are realized by switching between the downlink and uplink on a time division basis. For this reason, in the case of the TDD system, the downlink component carrier can also be expressed as “downlink communication timing in a component carrier.” The uplink component carrier can also be expressed as “uplink communication timing in a component carrier.” The downlink component carrier and the uplink component carrier are switched based on a UL-DL configuration as shown in FIG. 3. In the UL-DL configuration shown in FIG. 3, timings are configured in subframe units (that is, 1 msec units) for downlink communication (DL) and uplink communication (UL) per frame (10 msec). The UL-DL configuration can construct a communication system capable of flexibly meeting a downlink communication throughput requirement and an uplink communication throughput requirement by changing a subframe ratio between downlink communication and uplink communication. For example, FIG. 3 illustrates UL-DL configurations (Config 0 to 6) having different subframe ratios between downlink communication and uplink communication. In addition, in FIG. 3, a downlink communication subframe is represented by “D,” an uplink communication subframe is represented by “U” and a special subframe is represented by “S.” Here, the special subframe is a subframe at the time of switchover from a downlink communication subframe to an uplink communication subframe. In the special subframe, downlink data communication may be performed as in the case of the downlink communication subframe. In each UL-DL configuration shown in FIG. 3, subframes (20 subframes) corresponding to 2 frames are expressed in two stages: subframes (“D” and “S” in the upper row) used for downlink communication and subframes (“U” in the lower row) used for uplink communication.
Furthermore, as shown in FIG. 3, an error detection result corresponding to downlink data (ACK/NACK) is indicated in the fourth uplink communication subframe or an uplink communication subframe after the fourth subframe from the subframe to which the downlink data is assigned. In the TDD system, it is necessary to transmit response signals indicating error detection results corresponding to downlink data which has been indicated in a plurality of downlink communication subframes collectively in one uplink communication subframe. The number of downlink communication subframes at this time (which may also be called “bundling window”) is represented by M and an index of a downlink communication subframe corresponding to one uplink communication subframe is represented by m. For example, in UL-DL Configuration#2, error detection results on downlink data in four downlink communication subframes SF#4, 5, 6 and 8 of a first frame are indicated in uplink communication subframe SF#3 of a second frame. In this case, M=4, and SF#4, 5, 6 and 8 of the first frame correspond to m=0, 1, 2 and 3 respectively.
In TDD in an LTE-A system, a terminal receives downlink assignment control information via PDCCH and transmits a response signal over an uplink upon receiving downlink data. The following two methods are adopted as the method of transmitting the response signals.
Method 1 is a method (implicit signaling) whereby a terminal transmits a response signal using a PUCCH resource associated, in a one-to-one correspondence, with leading CCE index nCCE of CCE (Control Channel Element) occupied by PDCCH and index m of a downlink communication subframe corresponding to one uplink communication subframe (see PTL 1). Note that m is indexed in time sequence.
More specifically, the terminal first calculates parameter c={0, 1, 2, 3} that satisfies equation 1 in a magnitude relationship between leading CCE index nCCE occupied by PDCCH intended for the terminal (DL assignment) and Nc for each DL subframe m. Note that Nc in equation 1 is calculated according to equation 2. NDLRB in equation 2 is the number of downlink resource blocks and NRBSC is the number of subcarriers per resource block. The terminal then determines PUCCH resources n(1)PUCCH based on DL subframe m and calculated c according to equation 3 (see NPL 3). Note that N(1)PUCCH in equation 3 is an offset value for all PUCCH resources and is a value set in the terminal in advance.[1]Nc≦nCCE<Nc+1  (Equation 1)[2]Nc=max{0,└[NRBDL·(NscRB·c−4)]/36┘}  (Equation 2)[3]nPUCCH,j(1)=(M−m−1)·Nc+m·Nc+1+nCCE,m+NPUCCH(1)  (Equation 3)
As shown in FIG. 4, in TDD, PUCCH resource region 10 corresponding to PDCCH is divided for each c and each partial region corresponding to c is divided for each m. PUCCH resources for each c and each m are arranged from a frequency end direction toward the center direction of a component carrier in ascending order of m and in ascending order of c.
In the example of FIG. 4, when transmitting a response signal indicating an error detection result corresponding to downlink data indicated in a subframe with m=2, the terminal first calculates c from a magnitude relationship between leading CCE index nCCE occupied by PDCCH (DL assignment) intended for the terminal and Nc in virtual PUCCH resource region 50 that collects PUCCH resources corresponding to the subframe with m=2 (equation 1).
Next, the terminal arranges the response signal in PUCCH resource n(1)PUCCH (reference numeral 11 in FIG. 4) in actual PUCCH resource region 10 for the acquired c (c=0 in FIG. 4) (equation 3).
The range of c used here extends as the CCE index increases. A maximum value of the CCE index increases as the scale of the PDCCH region increases. Therefore, the greater the range of c used, the greater the PDCCH region becomes. The scale of the PDCCH region is defined by CFI (Control Format Indicator). For example, the PDCCH region is composed of three OFDM symbols when CFI=3, and it is therefore largest, whereas when CFI=1, the PDCCH region is composed of one OFDM symbol, and it is therefore smallest. Furthermore, CFI is dynamically indicated to the terminal for every subframe. Therefore, the PUCCH resource region is used more frequently when c is smaller. For this reason, the occupancy of control information in the PUCCH resource region increases as c decreases and decreases as c increases.
In the LTE-A system, a control signal may be transmitted using a plurality of PUCCH resources by applying transmission diversity or applying channel selection during carrier aggregation. In this case, as shown in equation 4 and FIG. 5, method 1 uses a predetermined PUCCH resource (reference numeral 11 in FIG. 5) and a PUCCH resource (reference numeral 12 in FIG. 5) adjacent to the PUCCH resource (which becomes nCCE+1 with respect to nCCE) in the actual PUCCH resource region. In other words, in the LTE-A system, an offset of +1 is added to the CCE index in the actual PUCCH resource region.[4]nPUCCH,j+1(1)=(M−m−1)·Nc+m·Nc+1+nCCE,m+1+NPUCCH(1)  (Equation 4)
Method 2 is a method (explicit signaling) whereby a base station indicates PUCCH resources to a terminal in advance and the terminal transmits a response signal using the PUCCH resources indicated in advance from the base station.
According to method 2, the base station can dynamically indicate to the terminal, information (ARI (Ack/Nack Resource Indicator)) indicating one PUCCH resource from among a plurality of PUCCH resources through DL assignment in advance. This makes it possible to dynamically switch between quasi-static PUCCH resources with a small number of bits. For example, when ARI has 2 bits, the base station can select one of four PUCCH resources.
In the LTE-A system, various devices are introduced as radio communication terminals such as M2M (Machine to Machine) communication, and the number of multiplexed terminals tends to increase by MIMO transmission techniques, and therefore the number of control signals transmitted from a base station to a terminal is considered to increase. For this reason, there may not be enough PDCCH regions which are regions in which PDCCHs used for control signals are mapped. When the base station cannot transmit control signals due to this shortage of resources, the base station can no longer assign data to the terminal. For this reason, the terminal can no longer use a PUSCH region to be used for data even if the PUSCH region is free and the system throughput may deteriorate.
As a method of solving this shortage of resources, studies are being carried out on the possibility of mapping control signals intended for terminals under a base station to a PDSCH region as well. This region in which control signals intended for terminals under the base station are mapped, that is, a region available to both control signals and data is called “enhanced PDCCH (ePDCCH) region.” Thus, the base station can transmit more control signals to terminals by providing ePDCCHs, and can thereby realize various kinds of control. For example, the base station can perform transmission power control on control signals transmitted to a terminal located near a cell edge or control of interference of transmitted control signals with another cell or control of interference of another cell with the cell formed by the base station.
In LTE, DL assignment instructing downlink data assignment (PDSCH) and UL grant instructing uplink data assignment are transmitted using PDCCH.
In LTE-Advanced, DL assignment and UL grant are transmitted also using ePDCCH in the same way as PDCCH. Studies are being carried out on the possibility that resources to which DL assignment is mapped and resources to which UL grant is mapped will be divided on the frequency axis in an ePDCCH region.
Methods 1 and 2 described above are defined as the method of determining PUCCH resources in a PUCCH resource region when DL assignment is indicated in a PDCCH region (hereinafter referred to as “PDCCH-PUCCH resource region”). Furthermore, in method 2, it is defined that a preset PUCCH resource is selected by dynamic ARI.
Here, when a PUCCH resource region in the case where DL assignment is indicated in an ePDCCH region (hereinafter referred to as “ePDCCH-PUCCH resource region”) is secured aside from a PDCCH-PUCCH resource region, the total amount of the PUCCH resource region increases. Especially when carrier aggregation is applied, PUCCH is transmitted using only one cell to avoid PAPR (Peak-to-Average Power Ratio) of the terminal from increasing. The cell is always PCell. PCell is generally a macro cell having a large coverage and high mobility is secured by a macro cell transmitting PUCCH. The capacity of PUCCH resources may be tight in the future due to not only introduction of ePDCCH but also introduction of carrier aggregation or introduction of M2M whereby many terminals perform data communication. Thus, in the LTE-A system, studies are being carried out on operation of causing ePDCCH-PUCCH resource regions to overlap with PDCCH-PUCCH resource regions.
As a method of determining ePDCCH-PUCCH resources, a method of instructing a preset offset value for an eCCE index using ARI or a method of defining a fixed value (equation 5) (see NPL 5). According to these methods, the base station first determines whether or not ePDCCH-PUCCH resources with offset value 0, that is, through implicit signaling, collide with PDCCH-PUCCH resources. When no collision occurs, the base station indicates ARI=0 to the terminal so as to use the ePDCCH-PUCCH resources. On the other hand, when collision occurs, the base station sequentially determines collision or no collision using ePDCCH-PUCCH resources with other offset values added thereto and indicates ARI or a fixed value corresponding to a non-colliding ePDCCH-PUCCH resource to the terminal.[5]nPUCCHE=nE-CCE+NPUCCH(1)+ARI  (Equation 5)
In ePDCCH, values such as 4, 8, 16, 32 are taken as scale NeCCE of one ePDCCH search space. However, values such as 2, 4, 8, 16 are taken as NeCCE in the case of a special subframe where the number of OFDM symbols to be used for the downlink is small (that is, the case where a configuration with a small number of OFDM symbols to be used for the downlink is set as a special subframe configuration in which the number of OFDM symbols to be used for downlink communication in a special subframe, a gap (downlink/uplink switching period) and a ratio of the number of OFDM symbols to be used for uplink communication are set). Furthermore, the base station can set ePDCCH search space sets which are different between terminals and these ePDCCH search space sets may have different sizes. The base station can further set a plurality of ePDCCH search space sets in one terminal and these ePDCCH search space sets may have different scales. The base station can further set N(1)PUCCH as start positions of different PUCCH resources for the respective ePDCCH search space sets.